1. Field of the Invention
This invention relates to an electron accelerator including a target exposed to the electron beam for the purpose of producing X-ray radiation and a collimator assembly limiting or defining the X-ray cone. More particularly, this invention relates to a collimator assembly for an electron accelerator, the collimator assembly comprising a collimator shielding block for blocking undesired X-rays and an insert piece or bushing inserted into the shielding block for defining the cone of the X-ray beam.
2. Description of the Pior Art
In radiation therapy, the X-ray cone issuing from an electron accelerator should have a dose rate or intensity of equal magnitude over its entire cross-section. This is necessary in order to be able to apply the minimum dose required for destroying the diseased tissue in the region of the seat of the disease, and at the same time to be able to spare the adjacent healthy tissue.
In electron accelerators the X-ray radiation is produced in a target by accelerated electrons. The dose rate in the X-ray cone being issued has a conical characteristic with a maximum in the direction in which the electron beam impinges upon the target. This maximum most often coincides with the symmetry axis of the collimator. From U.S. Pat. No. 4,157,475 it is known to obtain an equal intensity or dose rate distribution across the X-ray cone defined by the collimator assembly by installing a compensating member or flattening filter in the X-ray cone. This flattening filter has a conical construction. It is adapted in its form and in its radiation absorption properties to the characteristic of the dose rate at its point of application. Behind the flattening filter an X-ray cone is obtained having a dose rate of equal magnitude at a fixed tissue depth (for instance 10 cm) over the entire cross-section of the X-ray cone. At a lesser tissue depth (for instance 3 cm) the dose rate would increase from the interior toward the exterior, i.e. radially from the axis (non-uniform dose rate distribution). This could lead to a greater dose charge on the healthy tissue. In order to avoid the undesired excessive increase in the dose rate in the marginal region of the X-ray cone at a lesser tissue depth, it is known from U.S. Pat. No. 4,157,475 to roughen the interior wall surfaces of the collimator which limit the X-ray cone and define a conical passageway for the X-ray cone, in a direction transverse to the radiation direction. Particularly, steped grooves may be introduced into the interior wall surfaces, these grooves being arranged transversely to the radiation direction. The inner wall surfaces may be grooved such as to provide axially spaced relatively narrow annular ridge portions each conforming to the perimeter of the conical passageway and relatively wide annular intermediate wall portions providing groove regions such that the annular intermediate wall portions extend outwardly and are clear of the perimeter of the conical passageway. Each groove region may have a width dimension measured along the conical passageway which greatly exceeds the width of the annular ridge portions, but is not greater than about five millimeters. The ridge portions may have a slight pitch in the radiation direction in the manner of a screw thread. Due to the stepped grooves, there is a displacement of scatter locations to greater depths of the material. The quanta scattered at an acute angle are strongly absorbed in the edges of the grooves.
From U.S. Pat. No. 4,157,475 it is also known to admit the step-shaped grooves into a sleeve or bushing. This bushing is inserted in a fixed manner into the collimator shielding block. The bushing is not intended to be removed. The bushing consists of a material of low atomic number, such as, for example, iron, copper, or aluminum, whose atomic number is less than that of the collimator shielding block. Accordingly, the forward scattering is more pronounced in this material.
Due to the fixed bushing and in accordance with the fixed dimensions of the passageway, the collimator assembly has a certain maximum X-ray field size. Smaller fields may be obtained by means of adjustable X-ray shielding plates which are arranged behind the collimator assembly.
In medical applications tissues of various sizes are irradiated by X-rays. In such applications it seems desirable to have available a collimator assembly that allows for various maximum X-ray field sizes. Removing the total collimator assembly and replacing it by another one having different dimensions of its X-ray passageway is time consumming, tedious and expensive.
Compensating or flattening filters are known from Rev. Scient. Instr. 27, 1956, p. 584.